TOR (target of rapamycin) encodes a large Ser/Thr protein kinase which is structurally and functionally conserved in eukaryotic species from yeast to animals to plants. TOR is a catalytic subunit of a large protein complex and plays a central role in the regulation of cell growth, differentiation, proliferation, survival, protein synthesis and transcription by integrating signals from hormones, nutrients and the environment (De Virgilo 2006: Wullschleger 2006; Inoki 2006).
In yeast, TOR is encoded by two genes (TOR1 and TOR2), which have 80% overall amino acid similarity, and interacts with other regulatory proteins to form two distinct complexes, TOR complex 1 (TORC1) and TOR complex 2 (TORC2), respectively. TORC1 in yeast is inhibited by rapamycin and is responsive to nutrient and growth factor cues to regulate temporal cell growth and metabolism, while TORC2 is not inhibited by rapamycin and is implicated in the regulation of cytoskeleton and spatial aspects of cell growth such as cell polarity (De Virgilo 2006: Weissman 2001).
In contrast to yeast other eukaryotes possess only a single TOR gene but as in yeast, TOR exists in two distinct complexes: TORC1 and TORC2. In mammals and C. elegans. TORC1 is rapamycin sensitive, while TORC2 is insensitive. The Arabidopsis genome contains only one copy of TOR which is insensitive to rapamycin. It remains to be determined if there are two functional TOR complexes in plants analogous to other eukaryotes, (Loewth 2002: Wullschleger 2006)
The TOR protein possesses several different functional domains. The N-terminal 1200 residues consist of 20 HEAT repeats, which typically mediate protein-protein interactions. Following the HEAT repeat region is the focal adhesion target (FAT) domain which has been suggested to facilitate protein binding. The TOR protein further comprises the FRB domain, the binding site for the FKBP-rapamycin complex. The catalytic serine/threonine kinase domain, which contains a conserved lipid kinase motif, is adjacent to FATC domain, a putative scaffolding domain, which is located at the extreme carboxyl terminus. (Kunz 2000; Andrade 1995; Bosotti 2000: Zheng 1995).
TOR1 knockout yeast strains display small cell size, slow growth rate, and hypersensitivity to temperature and osmotic stress. In contrast, loss of TOR2 function arrests growth in the early G1 phase of the cell cycle. In mice, disruption of TOR causes lethality at embryonic day 5.5 (E5.5) and proliferation arrest in embryonic stem cells. The protein sequence of TOR from Arabidopsis shows 60% and 59% identity with TOR2 and TOR1 from yeast. Disruption of AtTOR leads to the premature arrest of endosperm and embryo development at a very early globular stage, (16-64 cells) (Barbet 1996; Gangloff 2004; Murakamie 2004; Menand 2002; Mahfouz 2006).
In yeast and mammals, inhibition of the TOR signaling pathway by nutrient starvation or rapamycin treatment leads to a rapid down regulation of 18S, 5.8S, 25S and 5S rRNA synthesis and subsequent transcription of the majority of the 130 ribosome protein genes. The rate of cell proliferation and growth directly depends on the rate of protein synthesis, and in turn, protein synthesis depends on ribosome biogenesis. Ribosome biogenesis requires coordination of the production of ribosome components, including 4 different rRNA molecules and 130 ribosome proteins. TOR is suggested be a central regulator for ribosome biogenesis through RNA polymerase I dependent modulation of 18S, 5.8S and 25S ribosomal RNA transcription (RNA polymerase II drives expression of ribosome proteins and RNA polymerase III controls 5SrRNA synthesis) (Warner 2001; Powers 1999).
Plant growth and development is highly dependent on environmental interactions that are pivotal for survival. Plants adjust growth and development in relation to nutrient availability, light intensity, water availability and additional environmental parameters. The mechanisms that are involved in the perception and transduction of these environmental cues are poorly understood (Mahfouz 2006: Deprost 2007).
There remains a need for methods of regulating plant growth and development.